This invention relates to polycrystalline ceramic fibers containing alumina and hafnia and optionally, other metal oxides, and novel intermediates for their production. The process of reducing the grain size of these compositions is also a part of the invention.
Claussen et al. and Kriven et al. present studies of a system of alumina, zirconia and hafnia. Advances in Ceramics, Vol. 3, Science and Technology of Zirconia, Heuer and Hobbs, ed. 1981 and Advances in Ceramics, Vol.12, Science and Technology of Zirconia II, Claussen Ruhle and Heuer, ed., 1984, respectively. L.M. Lopato et al. present a phase diagram for alumina/hafnia in Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy, February 1977, pp1331-1334. Int. J. High Technology Ceramics 2 (1986) 207-219 reports a study of systems of the oxides of aluminum, chromium, zirconium and hafnium. U.S. Pat. No. 4,665,040 discusses alumina/zirconia powders possibly containing hafnia as an impurity. None of these references recognizes or teaches anything about grain refinement in alumina/hafnia systems, and none of these references suggests alumina/hafnia fibers.
A number of references teach formation of ceramic fibers. Representative of these references are U.S. Pat. Nos. 4,125,406; 3,308,015; 3,992,498; 3,950,478; 3,808,015 and U.K. No. 1,360,198. None of these mention hafnia containing alumina fibers. One patent, U.K. No. 1,264,973 lists hafnium oxide as a possible inclusion in alumina fibers (page 1, line 76) but there is no teaching of such a fiber or any recognition of superior properties. None of the references suggests the preferred combination of oxides or the fibers of this invention.
Ceramics, in general, are formed by shaping a mixture of powders and binders and/or precursors into "green" forms such as fibers and other articles. These "green" articles are then heated carefully to remove volatile matter and are then sintered at high temperatures to remove porosity and densify their microstructures. However during this high temperature sintering process, the grain sizes in the ceramics increase with increasing time at temperature. Generally, the longer the times and/or higher the temperature, the larger the size of the grains. This invention provides ceramic fibers with the property of grain refinement or reduction during sintering. These fibers also exhibit excellent retention of strength after exposure to high temperatures.